The invention concerns the field of metering technology. It concerns a measuring circuit arrangement for voltage measurement and current measurement for electricity meters for direct connection.
In industrial applications, electronic electricity meters have already largely replaced electromechanical meters due to the advantages that they offer. Connection of these industrial meters as at present known is indirect, that is to say through current transformers or voltage transformers displaying the corresponding standard characteristics. Among the outstanding characteristics of electronic meters, particular mention should be made of the fact that they can be read at a distance, require no maintenance, are compact, highly accurate and capable of handling a wide range of current.
As a consequence of these clear advantages over electromechanical meters, the use of electronic electricity meters is also of growing importance for household metering. A fundamental difference between household meters and the industrial meters mentioned above can be seen in their mode of installation: household meters are typically connected direct, which means that they are connected directly to the conductors supplying the dependent load. Here, current measurement plays a key role, since it provides the galvanic isolation to the meter, and also because strict requirements in accordance with international standard IEC 1036 are imposed on these meters. The requirements relating to accuracy of current measurement in conformity with this standard include a low rate of amplitude error (<1%), a low rate of phase error (<0.30°) and a low rate of linearity error. Furthermore, direct current tolerance of Imax/π in conformity with the standard in question must be attained; this means that such electronic electricity meters must be able to continue to measure in accordance with the standard even when subjected to mixed current with a proportion of direct current≦Imax/π. Thus these electronic electricity meters for direct connection must not be brought to saturation even when subjected to a proportion of direct current of Imax/π, but even when subjected to such a direct current magnetic bias, must still be capable of representing the alternating proportion of the load current up to Imax,
Measuring circuit arrangements that use direct current tolerant current transformers with direct current tolerance up to ≦Imax/π are known, but these direct current tolerant current transformers are unable to cope with the maximum permissible phase shifts, as phase shifts of up to 5° between current and voltage measurement values can arise in them. This means that direct current tolerant current transformers, which are more expensive than conventional current transformers because of the special materials that have to be selected, have to be complemented with a suitable phase compensation arrangement involving yet further expense.
Correction of phase shift by means of a time delay, as described for instance in DE 196 39 410 A1, can only be of limited help here. As household meters are mass production products, economic considerations take on particular importance in the selection of all components. For this reason, the correction of phase shift installed in conventional meters is only possible within narrow limits by time delay, in addition to which, this correction by means of time delay is not coupled to frequency, so that such correction is in itself frequency-dependent, and therefore limited.
But as long as direct currents in connection with energy charging are not to be measured, but must simply be tolerated, the technology of sensors acting as transformers will continue—especially by reason of their simple, economical and sturdy construction. In this respect, the fact has also been taken into account until now that for a given current density, and due to their design, current transformers are larger, heavier and hence more expensive in comparison to other sensors, such as voltage transformers, for example.